Generally, styrenic resins have good processability and mechanical properties, and thus have been used in the production of exterior parts for many electronic goods. However, styrenic resins can combust and are not fire resistant. Thus, styrenic resins can ignite and spread fire from an external ignition source. Accordingly, styrenic resins are subject to various mandatory controls on flammability for safety reasons in many countries, including the United States, Japan, and Europe, and are required to have high flame retardancy to meet the Underwriter's Laboratories Standard for use in the housing of electric appliances.
One well known and widely used method for imparting flame retardancy is to add halogen-containing flame retardants and antimony compounds to rubber-reinforced styrenic resin. Such halogen-containing compounds include polybromodiphenyl ether, tetrabromobisphenol A, bromine-substituted epoxy compounds, chlorinated polyethylene and the like. Antimony trioxide and antimony pentoxide are mainly used as the antimony compounds.
Imparting flame retardancy by using halogen and antimony compounds together has the advantages of ease in obtaining flame retardancy and little deterioration of physical properties. However, hydrogen halide produced during processing can have a harmful effect on health. In particular, the main halogen-containing flame retardant, polybromodiphenyl ether, can produce very toxic gas such as dioxin or furan. Hence, efforts have focused on the development of methods for imparting flame retardancy without using a halogen-containing compound.
Because rubber-modified styrenic resin generally has little remaining char during combustion, it is hard to impart flame retardancy to it in its solid state (Journal of Applied Polymer Science, 1998, vol. 68, p. 1067). Therefore, adding a char forming agent to a rubber-modified styrenic resin can promote desirable flame retardancy.
U.S. Pat. No. 4,520,152 discloses flame retardant compositions including polyphenylene ether resin, a styrene resin and a cyclic phosphonate. However, it has a disadvantage in that it is necessary to use 20 wt % or more of polyphenylene ether.
Japanese Patent Laid-Open No. 11-199721 discloses a flame-retardant propylene resin composition including a propylene resin, a nitrogenous thermoplastic resin, phosphorus nitrogen flame retardant, and modified polypropylene. However, it fails to disclose a flame retardant method for a styrenic resin or styrenic resin-based blend.